Optical correlators determine the Fourier transform and inverse Fourier transform of complex images quickly with respect to counterpart statistical methods. Potential applications for optical correlators take advantage of the speed of the transformations by introducing a cross-correlation process. Displaying two images at the input plane introduces the cross-correlation process and the cross-correlation of images provides a measure of the similarity that allows images with similar features to be identified quickly. For example, joint-transform optical correlators perform comparisons of a reference fingerprint of a person against a database of fingerprints that contains the fingerprint as well as an identification of the person. A guide placed over a prism positions a finger or thumb of the person into alignment with the fingerprint comparison images in the database. A Helium-Neon (HE—NE) laser is directed through the prism and the person's fingerprint, or reference image, is displayed on one side of an optically addressed spatial light modulator (SLM) at the input plane of the optical correlator. The comparison images display in intervals or cycles on the second side of the SLM. After displaying the comparison image, a charge-coupled device (CCD) camera captures light intensities at the output plane of the optical correlator. One of the light intensities at the output plane represents a measure of the cross-correlation between the reference image and the comparison image. Upon comparing the measures of similarity, the closest fingerprint match to the person is determined.
The feasibility and speed of optical correlation is limited by the resolution of the coherent images at the input plane. Optical correlators quite frequently have identical, multi-element lenses (on the order of 150 lin pairs/mm) to Fourier transform the images displayed at an input plane onto the Fourier transform plane and to inverse Fourier transform the joint power spectrum onto the output plane. For less demanding applications, two projector objectives of large aperture having appropriate focal lengths can provide the optical system for non-optimized performance. SLM's provide a very practical, high-resolution display to project coherent images at the input plane. SLMs may include liquid-crystal, magnetic-optic, deformable mirror, or multiple quantum well implementations. However, fabrication processes and materials limit the resolutions available from the SLM as the yield rates go down and costs goes up nonlinearly for linear increase in horizontal resolution. The limited resolution, as well as the resolution of the input images, limits the amount of information, or number of images, that can be compared in one cycle of the optical correlator. For instance, the SLM may hold the two fingerprints, the reference image and the comparison image, in one cycle and then the reference image and a second comparison image in the next cycle. Even though the reference fingerprint must be compared against thousands or millions of fingerprints, and three fingerprints are taken for each person to provide an accurate identification, only two fingerprints are compared in one cycle of one joint-transform optical correlator. Therefore, the parallel-processing advantage of optical correlators may remain impractical for many applications, e.g. a routine traffic stop.